Method of controlling the spray droplet size of a spray nozzle apparatus for spray-drying applications, spray drying apparatus and nozzle therefore

ABSTRACT

A method of controlling the spray droplet size of a spray nozzle apparatus, in particular for the manufacturing of food powders, delivered to the spray nozzle comprises the following steps: a) providing a paste of a product to be sprayed by a spray nozzle; b) continuously determining the shear viscosity (η) of the product paste delivered to the spray nozzle; c) determining the mass flow rate (Qm) of the product paste delivered to the spray nozzle; d) determining the static pressure (P) of the product paste delivered to the spray nozzle; e) determining the density (p) of the product paste delivered to the spray nozzle; f) delivering the data obtained in steps b) to e) to a control device comprising a computer and a memory; g) calculating control data for adjusting the spray nozzle on the basis of the data obtained in steps b) to e) and on nozzle geometry parameters stored in the memory; h) sending the control data as control signals to a control means of the spray nozzle and adjusting the spray nozzle accordingly.

The present invention is directed to a method of controlling the spraydroplet size of a spray nozzle apparatus. It is further directed to aspray drying apparatus and a nozzle for such a spray drying apparatus.

The manufacturing of food powders is realized to a great extent by meansof spray drying. This process converts emulsions, suspensions anddispersions into powder. Spray nozzles create droplets, which are driedin hot air by evaporating water. The final powder quality, the finalpowder texture, the dryer process design, the drying efficiency, thewalls fouling behaviour, the operational safety, to name only a fewcharacteristics, are directly linked to the spray quality and thus theatomization process.

Known spray drying processes use atomization nozzles with fixedgeometries which cannot be adjusted inline to the process and productconditions during start-up, manufacturing operation and shut-down.Instead operators change the nozzle geometries prior to the productioncycle without the possibility to cover all the manufacturing situations.Such nozzles are chosen according to water tables. The manufacturing offood powders happens at significantly higher viscosities compared towater. Typical spray viscosities are within in a range comprised between1 to 300 mPas. There is no known nozzle apparatus capable to competewith such a wide range.

As an example, for dairy emulsions at concentrate total solids above50%, the concentrate viscosity increases in an exponential slope withfurther increase of total solids. This fact causes problems tospray-drying, if the concentrate viscosity exceeds a design limit of theatomizer nozzles. The design limit is described by means of an atomizerair-core break-down, which stops the creation of droplets and thus stopsefficient spray-drying and agglomeration of powders with a requiredtexture. Using prior art spray nozzle apparatus, air-core break downswithin atomizer nozzles cannot be determined visually, thus there iscurrently no means to operate the spray-drying process at its best pointwithout facing issues, such as powder blockages in cones and cyclones,wall fouling or atomizer beard formation, to name just a few issues.

Since the product and process conditions change from start-up toshut-down of the process the quality of the product achieved varies andproduct buildup can happen on the nozzle itself and on the walls of thespray-drying equipment, in particular on the walls of the dryingchamber, in cones of spray-dryers and cyclones, but also in theconveying ducts between the process units.

It is a first objective of the present invention to overcome theproblems identified with prior art equipment and methods and to enableto operate a spray-drying equipment at its best point and in the mosteconomical way, which involves to be able to spray material having thehighest possible total solids content and dry to obtain a dry powderhaving the maximum total solid content possible, without exceeding thedesign limit of the atomizers nozzles, which is triggered by theair-core break-down.

It is an object of the present invention to obtain a method ofcontrolling the spray droplet size of a spray nozzle apparatus whichallows controlling of the spray droplet size during the working process.This is particularly useful to have achieve a target spray droplet sizedistribution defined by the Sauter diameter and to keep a target dropletsize distribution constant even with changing product or materialproperties and changing process conditions.

This object is achieved by a method comprising the following steps:

a) providing a paste of a product to be sprayed by a spray nozzle;

b) continuously determining the shear viscosity (η) of the product pastedelivered to the spray nozzle;

c) determining the mass flow rate (Qm) of the product paste delivered tothe spray nozzle;

d) determining the spray pressure (P) of the product paste delivered tothe spray nozzle;

e) determining the density (p) of the product paste delivered to thespray nozzle;

f) delivering the data obtained in steps b) to e) to a control devicecomprising a computer and a memory;

g) calculating control data for adjusting the spray nozzle on the basisof the data obtained in steps b) to e) and on nozzle geometry parametersstored in the memory;

h) sending the control data as control signals to a control means of thespray nozzle and adjusting the spray nozzle accordingly.

The shear viscosity is used as input parameter to control the spraynozzle. It allows inline control of the spray nozzle and thus of thespray droplet size, via a stability criterion composed of the spray massflow rate Qm, the spray pressure P the product density (ρ) and theproduct viscosity (η).

This stability criterion ensures to operate the spray-nozzle withindesign limits, avoiding air-core break-downs in the swirl-chamber of thenozzle.

Furthermore, a consistent powder agglomeration is achieved in theproduct during a production cycle independent of the total amount ofsolid particles (TS) or independent of mass flow rate fluctuations. Bythis method, a process automation can be achieved through improved andsimplified reproducibility and reliability of product properties fordifferent spray-dryer types. A competitive production control isachieved by the inventive method via advanced design of final powderproperties like powder moisture, tap density, final agglomerate size andagglomerate stability. Due to the automation the production economy andprocess efficiency (best-point operation) is also enhanced.

In a preferred embodiment step b) of continuously determining the shearviscosity (η) of the product paste delivered to the spray nozzle iscarried out in a bypass to the product paste stream to the spray nozzle.The bypass has the advantage to measure the shear viscosity independentof the production mass flow rate to suit laminar flow conditions (atReynolds Re<2300), which allows the measurement of the shear viscosityaccording to the Differential Pressure Drop Method.

Preferably, the shear viscosity (η) of the product paste is determinedby the following steps:

b1) providing a constant feed-flow-rate of the product paste at laminarflow conditions;

b2) determining the mass flow of the product paste;

b3) delivering the product paste to a pressure-drop-meter anddetermining the pressure drop;

b4) calculating the shear viscosity (η) of the product paste on thebasis of the laminar mass flow determined in step b2), the pressure dropdetermined in step b3) and a known product density.

In case the step b) is carried out in a bypass the calculation in stepb4) considers also the bypass-mass-flow-rate.

Preferably, the determination of the pressure drop in step b3) iscarried out according to the differential pressure drop method.

This method enables inline recording of product shear viscosities e. g.of coffee and milk products before atomization with its specific productcharacteristics such as highly viscous (1-300 mPas) and shear-thinningflow behaviour (determination of 2^(nd) Newtonian plateau viscosity (η).The inline shear viscosity information is necessary to operate thecontrollable spray-nozzle inline in order to determine the best pointconfiguration of the atomizer and warn in case of design limit achieved.Thus, the inline differential pressure drop method allows a calibrationof the shear viscosity for Newtonian and in particular Non-Newtonianshear-thinning fluids based on laboratory rheometers.

Other techniques to measure the shear viscosity are eitherunderestimating or overestimating the predefined product shearviscosities of dairy and nutrition products (via laboratory rheometer).In particular for shear-thinning fluids, the frequency-based measuringtechnique, the Coriolis forced measuring method and thequartz-viscosimetry method do not give the possibility to determine the2nd Newtonian plateau viscosity of shear-thinning fluids due to the lackof information concerning the applied flow field of the method (and thusunknown shear rates).

Thus, inline recording of the so called second Newtonian plateauviscosity of Non-Newtonian food fluids is possible with the differentialpressure drop method and thus allows calibration with predefined productshear viscosity rheograms, which are found from laboratory rheometermeasurements.

According to a second aspect of the invention the object concerning thespray drying apparatus is achieved by the features of claim 7.

The spray drying apparatus according to the invention provides an inlineworking means to control spray droplet sizes during spray drying. Thespray quality can be judged in terms of the droplet size distributionand its corresponding droplet size mean diameter, i.e. the Sauterdiameter D₃₂.

The spray drying according to the invention helps to achieve thefollowing main manufacturing objectives: a minimum Sauter diameter forfastest and equilibrium water evaporation, an optimum powderagglomeration for consistent powder quality, an equilibrium powderparticle size distribution for consistent powder quality, theelimination of scorched particles for consistent powder quality, minimalpowder wall fouling and as a consequence reduced risk for cone, duct orconveying pipe powder blockages, minimal spray nozzle fouling andincreased dryer safety because of the elimination of dripping andelimination of scorched particles.

According to a third aspect of the invention a spray nozzle apparatus isprovided which comprises means for adjusting the nozzle chamber geometrybased on spray drying process parameters, like spray mass flow rate,spray pressure and product parameters, like product density, productshear viscosity which parameters are obtained or evaluated inline duringthe spray drying process in accordance with a method of the presentinvention.

Thus it is possible to adjust the nozzle geometry inline on the basis ofparameters responsible for the process yield and the quality of theproduct achieved. Furthermore the downtimes of a spray drying apparatusequipped with a spray nozzle apparatus according to the invention can bereduced since cleaning times are cut significantly thanks to minimisedequipment fouling.

The nozzle apparatus can be provided with an electric drive adjustingthe chamber geometry, the drive being controlled by a control device onthe basis of spray drying process parameters and product parameters asmentioned above.

To modify the chamber geometry, according to an advantageous embodimentof the invention, the apparatus comprises a plunger for adjusting thevolume of the nozzle swirl-chamber.

By moving the plunger into and out of the nozzle chamber by the electricdrive an adjustment of the height of the nozzle swirl-chamber isachieved. Thus by moving the plunger, the geometry of the nozzle chambercan be modified inline during the manufacturing process in relation tothe product and process parameters as mentioned above.

Movement of the plunger is achieved by the electric drive which in turnis controlled by a control device like a programmable circuit. Thiscircuit transmits control signals to the electric drive as a function ofthe above-mentioned parameters.

In order to achieve the above, according to an advantageous embodimentof the invention the electric drive comprises an electric motorrotatably driving an output shaft, the rotation being transformed in toa longitudinal motion of the plunger via a threaded engagement betweenthe output shaft and the plunger. Thus a mechanical stable and easy tohandle configuration is achieved.

According to an embodiment of the invention, a connecting sleeve isprovided which is releasably fixed to the electric drive and is equippedwith a longitudinal bore for rotatably accommodating a hollow shaftwhich transfers the rotating motion of an output shaft of the electricdrive to an adjusting pin driving the plunger axially into and out ofthe nozzle chamber.

The adjusting pin is provided with a longitudinally extending bore withan inner thread in engagement with an outer thread of the plunger suchthat a rotating motion of the adjusting pin is transformed into alongitudinal motion of the axially movable plunger.

According to an advantageous embodiment of the invention, the nozzlechamber is defined by a swirl chamber body being inserted into an innerchamber of a nozzle body, the nozzle body being releasably fixed to theconnecting sleeve mentioned above and the swirl chamber body is providedwith an opening channel which is arranged in correspondence to theorifice for entering the product material into the swirl chamber of theswirl chamber body. This material can for example be a paste for theproduction of dairy and nutrition products.

The swirl chamber can be provided with a helicoidally tightening guidingface for accelerating the paste into the direction of the nozzle orificeto output the material droplets with high speed. Since the material isincompressible, the cone angle of the spray cone and the dropletdiameter can be modified according to the product and process parametersinline during the manufacturing process of the product to be achieved bythe adjustable movement of the plunger within the swirl chamber.

According to an advantageous embodiment of the invention, the orificefor introducing the material into the nozzle chamber extends radially tothe longitudinal axis of the nozzle and the product material is beingtransferred to the nozzle via a tube being connected to the orifice.

To enable a basic modification of the output characteristics of thespray nozzle, the nozzle body is equipped with a releasably mountedorifice plate such that the opening diameter of the nozzle orifice isvariable by replacing the orifice plate by a different diameter orificeplate.

According to a preferred characteristic, a cone angle of a spray mistproduced by product droplets and the droplet size are variable byaxially moving the plunger relative to the nozzle chamber.

In the following the invention will be described in further detail bymeans of an embodiment thereof and the appended drawings.

FIG. 1 shows a partial sectional side view of an embodiment of a spraynozzle apparatus according to the invention;

FIG. 2 shows a cross sectional view of a hollow shaft of the spraynozzle apparatus of FIG. 1;

FIG. 3 shows a partial sectional view of an adjusting pin;

FIG. 4 shows a front view of the swirl chamber body of the spray nozzleapparatus of FIG. 1;

FIGS. 5 and 5A depict a side view and a front view (in the direction ofarrow A) of the plunger of the spray nozzle apparatus of FIG. 1;

FIG. 6 is a flow chart of the process control method according to theinvention;

FIG. 7 is a flow chart of the differential pressure drop method;

FIG. 8 shows a principle of a measuring apparatus for the differentialpressure drop method and

FIG. 9 shows an example of a dimensionless correlation between thedroplet size of the spray and the geometry, process and productparameters.

The spray nozzle apparatus 1 according to FIG. 1 comprises an electricdrive 2 provided with an interface (such as a Profibus interface) and apower supply (such as a 24V-DC power supply) at 3 and an electric motor4 including a transmission connected with 3.

The electric motor 4 drives an output shaft 5 in a rotating manner. Theoutput shaft 5 extends into a longitudinally extending inner bore 6 of ahollow shaft 7 which is depicted in more detail in FIG. 2.

The hollow shaft 7 is rotatably accommodated in a longitudinallyextending inner bore 8 of a connecting sleeve 9 which can be fixed tothe housing of transmission 4 by bolts 10.

The inner bore 6 of the hollow shaft 7 is equipped with an inner thread11 which can be brought into a threaded engagement with an outer thread12 provided on an end piece of an adjusting pin 13—shown in more detailin FIG. 3—which can be inserted into the inner bore 6 of the hollowshaft 7.

Opposite to the threaded terminal end 12 of the adjusting pin 13 thereis provided a receiving section of the adjusting pin 13, which is formedwith an inner bore 14 equipped with an inner thread 15.

The inner thread 15 of the adjusting pin 13 serves to be brought into athreaded engagement with an outer thread 16 of a plunger 17 more clearlyshown in FIGS. 5 and 5A.

As can be seen from FIGS. 5 and 5A, the plunger 17 comprises an outercircumferential surface section 18 with a helicoidally shaped crosssection corresponding to the shape and size of a receiving section 19 ofa swirl chamber body 20 accommodated in a nozzle body 23 which ismounted to the connecting sleeve 9 as shown in FIG. 4.

The swirl chamber body 20 comprises a lateral or tangential inletchannel 21 for introducing paste material or the like into the swirlchamber 22 of the swirl chamber body 20.

Material to be transported through the inlet channel 21 into the swirlchamber 22 can enter the nozzle body 23 via a first orifice 24 or inletorifice which extends radially to the common longitudinal axis 28 of thenozzle body 23 and the connecting sleeve 9. To this end there is a tube25 connected to the first orifice 24 of the nozzle body 23 defining aninlet opening of the apparatus 1.

Paste or paste like material delivered to the nozzle body 23 via thetube 25 enters the nozzle body 23 via the first orifice 24 and entersthe swirl chamber 22 via the inlet channel 21.

The swirl chamber 22 is equipped with an axially extending through holehaving an inner circumferential surface section with a helicoidallyshaped cross section, thus forming a helicoidal, spiral-type guidingface that serves to accelerate the material into the direction of asecond orifice 26 or nozzle orifice of the nozzle body 23 defining anoutlet opening of the apparatus 1. An orifice plate 27 is providedbetween the axial outlet of the swirl chamber 22 and the second orifice26 by which orifice plate 27 the opening angle of the spray cone can bebasically adjusted.

FIG. 1 shows the plunger 17 closing the first orifice 24. Driving themotor 4 makes the hollow shaft 7 rotate and thus also makes theadjusting pin 13 rotate about its longitudinal axis. The plunger 17 isconnected to the inner thread 15 of the adjusting pin 13 via the outerthread 16 and can only execute a movement relative to the swirl chamberbody 20 along the longitudinal axis 28 of the plunger 17 but can notrotate relative to the swirl chamber body 20. Thus a rotation of theadjusting pin 13 is transformed in to an axial movement of the plunger19 relative to the swirl chamber body 20.

By this movement of the plunger 18 the axial width of the first orifice24 and the geometry of the swirl chamber 22 and thus the nozzle chambercan be modified. Since the electric drive 2 is controlled by process andproduct parameters which in turn are obtained or evaluated inline duringthe manufacturing process of the powder to be achieved, the controltakes place inline with the manufacturing process of the powder. Toachieve this, the control circuit provides the electric drive 2 withsignals such that the plunger 17 is being moved axially in the directionof the longitudinal axis 28 as shown in FIG. 1. By this movement of theplunger 17 the spray droplet size of the sprayed material to be atomizedcan be adjusted towards the minimum Sauter diameter possible for a givenset of input parameters.

Measuring these input parameters inline with the production process ofthe powder according to the method of the invention allows adjusting ofthe droplet size towards the minimum Sauter diameter possible inline andthus makes it possible to consider the complete range of sprayviscosities during the production process of the powder to be produced.

The product paste entering the swirl chamber through the inlet channel21 follows a helicoidal and spiral way due to the spiral-type crosssection design of the swirl chamber in a combined circumferential andaxial direction towards the nozzle orifice 26. This design acceleratesthe traveling speed of the product paste flow in the swirl chamber,provided that the mass flow of the product paste is constant. Theproduct paste is leaving the spray nozzle through the orifice plate 27and the nozzle orifice 26 as a cone-shaped film 29 with a cone tip angleα wherein the film 29 atomizes into droplets forming a spray mist. Thecone tip angle α is directly proportional to the traveling speed of theproduct paste in the nozzle orifice 26, i.e. the higher the travelingspeed is, the larger the cone tip angle becomes and the smaller thedroplets size.

A cone tip angle α of 0° generates no atomization and, in a realizedexample, a cone tip angle α of 100° generates droplets having aSauter-diameter of D₃₂=30 μm. The wider the cone tip angle α is, thesmaller the droplets become so that the droplet size can be controlledby the cone tip angle α and thus by the traveling speed of the productpaste in the nozzle orifice 26

FIG. 6 is a flowchart of the process control method according to thepresent invention. The product paste in FIG. 6 indicated as“concentrate” is delivered to a dosing point 30, which leads a part ofthe product paste stream into a bypass line 32. The majority of theproduct paste stream is directed into a main product paste line 34. Thebypass line 32 is redirected into the main product paste line 34 at aline junction 36 downstream of a differential pressure drop measuringapparatus 38 provided in the bypass line 32.

Downstream of the line junction 36 a mass flow meter 40, a density meter42 and a spray pressure probe 44 are provided in the main product pasteline. Downstream of the spray pressure probe 44 the main product pasteline 34 enters the spray nozzle apparatus 1 shown in FIG. 1 through tube25. The product paste delivered to the spray nozzle apparatus 1 is thensprayed into a spray drying chamber 46.

The differential pressure drop measuring apparatus 38 determines theshear rate and the shear viscosity η of the product paste delivered tothe spray nozzle. The data of the shear rate and shear viscosity η aredelivered from the differential pressure drop measuring apparatus 38 toa control device (SPS-control) 48. In the same manner, the product pastemass flow rate Q_(m) determined in the mass flow meter 40, the productpaste density ρ determined in the density meter 42 and the spraypressure P of the product paste determined in the spray pressure probe44 are also delivered to the control device 48.

Control device 48 comprises a computer which calculates an outputcontrol parameter based on the above data delivered to the controldevice 48 and on the basis of known spray nozzle geometry parametersstored in a memory of the control device 48. The output controlparameter is delivered to the spray nozzle apparatus 1 in order toadjust the swirl chamber piston 17 to a calculated position in order toobtain a desired swirl chamber volume.

The following equations 1-7 describe the solving procedure how tocontrol the plunger position (given with h_(sc)) based on a change inthe paste shear viscosity η.

Accordingly the solving procedure is applied for a change in mass flowrate Qm and paste density ρ.

Universal Massflow-Characterisation of Pressure Swirl Nozzle Flows:

$\begin{matrix}{\frac{Qm}{\eta \; d_{sc}} = {2.1844\left( \frac{d_{or}}{d_{sc}} \right)^{1.2859}\left( \frac{h_{sc}}{d_{sc}} \right)^{0.4611}\left( \frac{\sqrt{P\; \rho}d_{sc}}{\eta} \right)^{0.9140}}} & (1)\end{matrix}$

-   The relation between spray pressure P and axial position of the    plunger (given with h_(sc)) is derived for the example of a shear    viscosity change from η_(old) to η_(new):

$\begin{matrix}{\frac{\eta_{new}}{\eta_{old}} = {\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{0.4611}\left( \frac{\eta_{new}}{\eta_{old}} \right)^{0.9140}\left( \frac{P_{old}}{P_{new}} \right)^{\frac{0.9140}{2}}}} & (2)\end{matrix}$

-   Solved for the spray pressure ratio:

$\begin{matrix}{\frac{P_{old}}{P_{new}} = {\left( \frac{\eta_{new}}{\eta_{old}} \right)^{\frac{1 - 0.9140}{0.4570}}\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{\frac{- 0.4611}{0.4570}}}} & (3)\end{matrix}$

-   In order to find a direct relation between plunger position h_(sc)    and shear viscosity η, the spray pressure ratio has to be found from    another equation, see equations 4-6 below:-   Universal Spray Droplet Size Characterisation of Pressure Swirl    Nozzle Sprays:

$\begin{matrix}{\frac{D_{32,{global}}}{d_{sc}} = {1.0798\mspace{14mu} {Re}^{- 0.2987}{{We}^{- 0.1709}\left( \frac{h_{sc}}{d_{sc}} \right)}^{- 0.0772}\left( \frac{d_{or}}{d_{sc}} \right)^{0.9534}}} & (4)\end{matrix}$

-   Again, one can derive the Spray Pressure Ratio with the consistency    conditions that D_(32-global-old) and D_(32-global-new) remain    constant:

$\begin{matrix}{\frac{D_{32,{global},{old}}}{D_{32,{global},{new}}} = {1 = {{\left( \frac{{Re}_{old}}{{Re}_{new}} \right)^{- 0.2987}\left( \frac{{We}_{old}}{{We}_{new}} \right)^{- 0.1709}\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{- 0.0772}} = {\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{- 0.2987}\left( \frac{\eta_{old}}{\eta_{new}} \right)^{0.2987}\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{0.2987}\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{0.1709 \cdot 2}\left( \frac{h_{{sc},{old}}}{h_{{sc},{new}}} \right)^{- 0.0772}}}}} & (5)\end{matrix}$

-   And hence the solution, how to control the plunger height h_(sc,new)    based on a current position h_(sc,old):

$\begin{matrix}{\frac{h_{{sc},{new}}}{h_{{sc},{old}}} = \left( \frac{\eta_{new}}{\eta_{old}} \right)^{- 1.1289}} & (6)\end{matrix}$

-   Combining equations 3 and 6 one receives the solution, how to    control the spray pressure:

$\begin{matrix}{\frac{P_{new}}{P_{old}} = \left( \frac{\eta_{new}}{\eta_{old}} \right)^{0.9508}} & (7)\end{matrix}$

FIG. 7 is a flowchart of the differential pressure drop method asapplied in the differential pressure drop measuring apparatus 38. A feedpump 50 is provided in the bypass line 32 downstream of dosing point 30.The feed pump 50 ensures a constant feed-flow-rate in the differentialpressure drop measuring apparatus 38 to enable shear rates which coverthe second Newtonian viscosity plateau. Downstream of the feed pump 50 amass flow meter 52 is provided through which the product paste in thebypass line 32 is directed into a pressure drop meter 54. The shearviscosity (η) of the product paste in the bypass line 32 is calculatedfrom the mass flow measured in the mass flow meter 52, the known productdensity of the product paste and the pressure drop measured in thepressure drop meter 54. This calculation is either made in a computer(not shown) of the differential pressure drop measuring apparatus 38 or,the respective data are delivered to the control device 48 and the shearviscosity η is calculated in the computer of the control device 48. Inorder to consider the fact that the pressure drop is measured in abypass line 32 the bypass mass flowrate is adjusted by the feed pump 50until the shear-rate is such, that the second Newtonian plateauviscosity can be measured by the pressure drop-meter 54 within laminarflow conditions.

In the present example the dosing point 30 regulates the bypass flowrate to keep the bypass flow pressure <20 bar at laminar flowconditions, e.g. flow rates <1000 kg/h.

FIG. 8 shows the principle of a measuring apparatus (pressure dropmeter) for the differential pressure drop method for determination ofthe second Newtonian plateau viscosity using three independent pressuredrop recordings at three different shear-rates.

The pressure drop meter 100 comprises a tube having a fluid inletsection 102 and a fluid outlet section 104 and three pressure dropmeasuring sections 106, 108, 110 provided between the inlet section 102and the outlet section 104. The first pressure drop measuring section106 which is close to the inlet section 102 has a first internaldiameter d₁ and a first axial length l₁. A first differential pressuremeter 112 measuring a first pressure drop Δp₁ is connected to the firstpressure drop measuring section 106 in a commonly known matter whereinthe axial distance L₁ between the two static pressure measuring openingsin the wall of the first pressure drop measuring section 106 issubstantially equal to the length l₁ of the first pressure dropmeasuring section 106.

The second pressure drop measuring section 108 is provided downstream ofthe first pressure drop measuring section 106. The internal diameter d₂of the second pressure drop measuring section 108 is smaller than thediameter d₁ of the first pressure drop measuring section. The length l₂of the second pressure drop measuring section 108 is shorter than thelength of the first pressure drop measuring section 106. The secondpressure drop measuring section 108 comprises a second differentialpressure meter 114 measuring a second pressure drop Δp₂ wherein thedistance L₂ between the two static pressure measuring openings in thewall of the second pressure drop measuring section 108 is shorter thanthe distance L₁ of the first differential pressure meter 112.

A third pressure drop measuring section 110 is provided downstream ofthe second pressure drop measuring section 108 and the third pressuredrop measuring section 110 opens into the outlet section 104. Theinternal diameter d₃ of the third pressure drop measuring section 110 issmaller than the diameter d₂ of the second pressure drop measuringsection 108 and the length l₃ of the third pressure drop measuringsection is shorter than the length l₂ of the second pressure dropmeasuring section. The third pressure drop measuring section 110comprises in a commonly known manner a third differential pressure meter116 measuring a third pressure drop Δp₃. The distance L₃ between the twostatic pressure measuring openings in the wall of the third pressuredrop measuring section 110 is shorter than the distance L₂ of the seconddifferential pressure meter 114.

The differential pressure drop meter 100 allows the measurement of threeindependent pressure drop recordings of the first, the second and thethird differential pressure drop meters. Utilizing these threedifferential pressure drop probes in series, a single mass flow ratecauses three increasing wall shear rates with the decreasing tubediameter.

The following equation 8 is used to calculate the shear viscosity η forlaminar tube flows (Re<2300), applied to all 3 differential pressuresΔp₁, Δp₂ and Δp₃ (respectively measured at 112, 114 and 116, FIG. 8), byreplacing Δp_(i) and the corresponding tube dimensions (R_(i) and L_(i))in equation 8:

Only, if the shear viscosity η_(i) is equal (η₁=η₂=η₃) between the 3differential pressures, the 2^(nd) Newtonian shear viscosity is foundand used e.g. in equation 1 and 7, etc. . . .

$\begin{matrix}{\eta_{i} = \frac{{\pi \cdot R_{i}^{4} \cdot \Delta}\; {p_{i} \cdot \rho}}{8 \cdot {Qm} \cdot L_{i}}} & (8)\end{matrix}$

with following definitions of symbols:

-   R_(i): tube radius (R₁, R₂ and R₃) in [m]-   Δp_(i): tube pressure drop (Δp₁, Δp₂ and Δp₃) in [Pa]-   ρ: product density in [kg/m3]-   Qm: mass flow rate in [kg/s]-   L_(i): tube length (distance L₁, L₂ and L₃) in [m]

FIG. 9 shows an example of a dimensionless correlation between thedroplet size of the spray and the geometry, process and productparameters. The droplet size D_(32, global) is the Sauter diameter ofthe spray droplets. The dimensionless Weber number We and the Eulernumber Eu represent the four input process parameters: Spray mass flowrate Q_(m), static spray pressure P, product density ρ and product shearviscosity η. The geometry of the spray nozzle is described with theparameters h_(sc), d_(sc), d_(or) and b_(ch). These abbreviations areexplained in table 1 below.

-   The Sauter diameter D_(32, global) was measured by phase-doppler    anemometry (PDA) of the droplets sprayed by the spray nozzle    apparatus.-   The measured Sauter diameter D_(32, global) was correlated to the    corresponding geometry, process and product parameters, which were    varied in the frame of the PDA measurements to achieve a correlation    as shown in FIG. 9.

TABLE 1 Abbreviations and formula Symbol, Abbreviation Description UnitsD_(32,global) Global Sauter diameter as found [m] from PDA measurementsof spray d_(sc) Swirl chanber diameter [m] (smallest diameter of swirlchanber spiral) h_(sc) Swirl chamber height [m] (axial height of swirlchamber) d_(or) Orifice diameter [m] (diameter of opening made inorifice plate) b_(ch) Width of swirl chamber inlet [m] channel (smallestwidth of inlet channel which leads into the swirl chamber) We Webernumber  ${We} = \frac{\rho_{liquid}u_{bulk}^{2}d_{orifice}}{\sigma_{liquid}}$— Eu Euler number   ${Eu} = \frac{P}{\rho_{liquid}u_{bulk}^{2}}$ — ReReynolds number   ${Re} = \frac{\rho_{liquid}u_{bulk}h_{sc}}{\mu}$ —u_(bulk) Bulk velocity at swirl chamber inlet  $u_{bulk} = \frac{Qm}{\rho_{liquid}h_{sc}b_{ch}}$ [m/s] Qm Mass flowrate [kg/s] P Spray pressure [Pa] ρ_(liquid) Liquid density [kg/m³]η_(liquid) Liquid chear viscosity [Pas] σ_(liquid) Surface tension [N/m]PDA Phase-Doppler Anemometry —

The invention should not be regarded as being limited to the embodimentshown and described in the above but various modifications andcombinations of features may be carried out without departing from thescope of the following claims.

1. A method of controlling the spray droplet size of a spray nozzleapparatus delivered to the spray nozzle, the method comprises thefollowing steps of: a) providing a paste of a product to be sprayed by aspray nozzle; b) continuously determining the shear viscosity of theproduct paste delivered to the spray nozzle; c) determining the massflow rate of the product paste delivered to the spray nozzle; d)determining the static pressure of the product paste delivered to thespray nozzle; e) determining the density of the product paste deliveredto the spray nozzle; f) delivering the data obtained in steps b) to e)to a control device comprising a computer and a memory; g) calculatingcontrol data for adjusting the spray nozzle on the basis of the dataobtained in steps b) to e) and on nozzle geometry parameters stored inthe memory; and h) sending the control data as control signals to acontrol means of the spray nozzle and adjusting the spray nozzleaccordingly.
 2. The method according to claim 1, wherein the step b) ofcontinuously determining the shear viscosity of the product pastedelivered to the spray nozzle is carried out in a bypass to the productpaste stream to the spray nozzle.
 3. The method according to claim 1,wherein the shear viscosity of the product paste is determined by thefollowing steps: b 1) providing a constant feed-flow-rate of the productpaste; b2) determining the mass flow of the product paste; b3)delivering the product paste to a pressure-drop-meter and determiningthe pressure drop; and b4) calculating the shear rate and shearviscosity of the product paste on the basis of the mass flow determinedin step b2), the pressure drop determined in step b3) and a knownproduct density.
 4. The method according to claim 2, wherein thecalculation in step b4) considers also the bypass-mass-flow-rate.
 5. Themethod according to claim 3, wherein the determination of the pressuredrop in step b3) is carried out according to the differential pressuredrop method.
 6. The method according to claim 5, wherein the adjustmentof the spray nozzle in step h) is carried out by changing the volume ofa swirl chamber provided in the spray nozzle.
 7. A spray dryingapparatus comprising a spray nozzle apparatus comprising a spray nozzleprovided with a nozzle orifice for outputting spray droplets of aproduct to be dried and an inlet orifice for transferring the productinto a nozzle chamber and an apparatus for adjusting the size of theoutputted droplets inline during the spray process on the basis of thecontrol data calculated by determining the shear viscosity and mass flowrate of product delivered to the spray nozzle.
 8. The spray dryingapparatus of claim 7, wherein the apparatus comprises a member foradjusting the nozzle chamber geometry based on spray drying processparameters and product parameters obtained inline during the spraydrying process.
 9. The spray drying apparatus according to claim 8,comprising an electric drive adjusting the chamber geometry, the drivebeing controlled by a control device on the basis of spray dryingprocess parameters and product parameters.
 10. The spray dryingapparatus according to claim 8, comprising a plunger adjusting the sizeof the inlet orifice and/or the volume of the nozzle chamber.
 11. Thespray drying apparatus according to claim 10, wherein the plunger ismovable into and out of the nozzle chamber by the electric driveadjusting the inlet orifice width and/or the height of the nozzlechamber.
 12. The spray drying apparatus according to claim 10, whereinthe electric drive comprises an electric motor rotatingly driving anoutput shaft, the rotation being transferred to a longitudinal motion ofthe plunger via a threaded engagement between the output shaft and theplunger.
 13. The spray drying apparatus according to claim 9, comprisinga connecting sleeve being releasably fixed to the electrical drive andproviding a longitudinal bore for rotatingly accommodating a hollowshaft which transfers the rotating motion of an output shaft of theelectrical drive to an adjusting pin driving a plunger into and out ofthe nozzle chamber.
 14. The spray drying apparatus according to claim13, wherein the adjusting pin is provided with a longitudinallyextending axial bore with an inner thread in engagement with an outerthread of the plunger such that a rotating motion of the adjusting pinis transferred to a longitudinal motion of the axially movable plunger.15. The spray drying apparatus according to claim 13, wherein the nozzlechamber is provided by a swirl chamber body being inserted into an innerchamber of a nozzle body, the nozzle body being releasably fixed to theconnecting sleeve and the swirl chamber body being provided with anopening channel which is arranged in correspondence to the inlet orificefor entering the material into a swirl chamber of the swirl chamberbody.
 16. The spray drying apparatus according to claim 15, wherein theswirl chamber is provided with a helicoidally tightening guiding facefor accelerating the product into the direction of the nozzle orifice.17. The spray drying apparatus according to claim 8, wherein the inletorifice extends radially to the longitudinal axis of the nozzle and theproduct being transferred to the nozzle via a tubing being connectedwith the inlet orifice.
 18. The spray drying apparatus according toclaim 8, wherein the nozzle orifice is equipped with a releasablymounted orifice plate such that the opening diameter of the nozzleorifice is variable by replacing the orifice plate by a differentdiameter orifice plate.
 19. The spray drying apparatus according toclaim 10, comprising a cone angle of a spray mist produced by productdroplets and the droplet size are variable by axially moving the plungerrelative to the nozzle chamber.